Dynamic Light Scattering Study of Temperature and
pH Sensitive Colloidal Microgels
Valentina Nigroa, Roberta Angelinib,c, Monica Bertoldod, Valter
Castelvetroe, Giancarlo Ruoccoc,f, Barbara Ruzickab,c
aDipartimento di Scienze, Università degli Studi Roma Tre, 00146 Roma, Italy. bIstituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche
(IPCF-CNR), UOS Roma, Pz.le Aldo Moro 5, I-00185 Roma, Italy.
cDipartimento di Fisica, Sapienza Universit`a di Roma, Pz.le Aldo Moro 5, I-00185, Italy. dIstituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche
(IPCF-CNR), Area della Ricerca, Via G.Moruzzi 1, I-56124 Pisa, Italy.
eDipartimento di Chimica e Chimica Industriale, Universit`a di Pisa, via Risorgimento
35, I-56126 Pisa, Italy.
fCenter for Life Nano Science, IIT@Sapienza, Istituto Italiano di Tecnologia, Viale
Regina Elena 291, 00161 Roma, Italy.
Abstract
Microgels composed of Interpenetrated Polymer Networks (IPN) of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc) dispersed in water have been investigated through dynamic light scattering. The study of the temperature, concentration and pH dependence of the relaxation time has highlighted the existence of a thermoreversible transition corresponding to the swollen-shrunken volume phase transition. The presence of PAAc intro-duces an additional pH-sensitivity with respect to the temperature-sensitivity due to PNIPAM and leads to interesting dierences in the transition process at acid and neutral pH.
Keywords:
1. Introduction
Colloidal systems have long been the subject of intense research [] ei-ther for theoretical implications and for technological applications. They are very good model systems for understanding the general problem of dynamic arrest, thanks to their larger versatility with respect to atomic and molecu-lar glasses. Moreover, thanks to their dimensions, colloidal systems can be
easily investigated through conventional techniques, such as dynamic light scattering (DLS) and optical microscopy. The control of their interparticle potential by tuning external parameters such as packing fraction, waiting time and ionic strength, has given rise to exotic phase diagrams with dif-ferent phases and arrested states and unusual phase transitions. In the last decades an interesting new class of colloidal systems that allows to modu-late the interaction potential through temperature and/or pH, usually not relevant parameters, has been identi ed: aqueous dispersions of microgels. Microgels are microscopic particles of hydrogels largely studied in the last years because of their versatility and high sensitivity to external stimuli [1 5]. In this framework responsive microgels that can reversibly change their volume in response to slight changes in the properties of the medium, such as pH, temperature, electric eld, ionic strength, solvent, external stress or light, are particularly attractive smart materials. On the other hand due to their interesting properties they nd many applications in a lot of dier-ent elds such as in agriculture, construction, cosmetic and pharmaceutics industries, in arti cial organs and tissue engineering [2, 4, 611].
One of the most studied responsive microgel is based on the poly(N-isopropylacrylamide) also known as PNIPAM, a thermo-sensitive polymer. PNIPAM microgels were investigated for the rst time in 1986 by Robert Pelton and Philip Chibante [12], since then they have been widely stud-ied both experimentally and theoretically and a clear picture of preparation, characterization and applications has been provided [1, 2, 4, 5, 13]. PNIPAM microgels responsiveness is strongly dependent on the thermo-sensitivity of PNIPAM that presents a Lower Critical Solution Temperature in water at about 305 K. At room temperature indeed, it is found in a swollen state, the polymer is hydrophilic and a great amount of water is retained; by in-creasing temperature above 308-311 K the polymer becomes hydrophobic, water is completely expelled and a shrunken state is found giving rise to a volume phase transition (VPT). As a result, a microgel based on PNIPAM undergoes a characteristic VPT as well due to the temperature sensitivity of the interparticle potential hence to the delicate balance between hydrophilic-ity and hydrophobichydrophilic-ity [14, 15]. Phase diagram [16, 17] and important details on the gel structure of PNIPAM microgels near the volume phase transition have been obtained [1821]. It has been recently shown that the microgel swelling/deswelling behavior can be strongly aected by concentra-tion [22, 17], by solvents [23] and by synthesis procedure such as growing number of cross-linking points [24, 25], dierent reaction pH conditions [26]
or by introducing additives into the PNIPAM network [27, 28].
In this context PNIPAM microgels containing another specie as comonomer or interpenetrated polymer result to be even more interesting systems since they retain the main properties of the constituent polymer. In particular, adding acrilic acid (AAc) to PNIPAM microgel provides a pH-sensitivity to the system that leads to a more complex phase behavior. In this way a thermo and pH-sensitive system is obtained. As in the case of pure PNIPAM micro-gel a volume phase transition with temperature is observed, although with a remarkable reduction in the swelling capability as the AAc concentration is increased [29, 30]. Indeed the volume phase transition of these microgels is strongly dependent on the eective charge density controlled by the content of AAc monomer [29, 30], on the pH of the suspension [3134] and on the salt concentration [31, 35]. In this framework the synthesis procedure plays an important role. Indeed, AAc can be incorporated into PNIPAM either by random copolymerization (PNIPAM-co-AAc) [31, 32, 3439] or by polymer interpenetration (IPN PNIPAM-PAAc) [29, 33, 4043]. In the rst case par-ticles are composed of a single network of both monomers, with properties dependent on the monomer ratio [31]. Conversely, in the second case micro-gels are made of two interpenetrated homopolymeric networks of PNIPAM and PAAc, with the same independent response as the two components to external stimuli [41]. This important characteristic of interpenetrating poly-mer network (IPN) microgels makes the mutual interference between the temperature-responsive and pH-responsive polymers largely reduced making the temperature dependence of the VPT unchanged with respect to the case of pure PNIPAM microgel (close to physiological temperature) and the IPN microgel more suitable for applications in controlled drug release and sen-sors [6, 44]. However investigations on IPN PNIPAM-PAAc microgels are scarce and only a preliminary temperature-concentration phase diagram has been reported up to now [29], leaving unexplored the dependence on pH, ionic strength and crosslink density. A few investigations through Dynamic Light Scattering (DLS), rheology and microscopic techniques have con rmed the existence of the VPT transition [33, 4043]. Nevertheless the scenario is far from being completely clear. Furthermore other open issues are related to the possibility to vary the softness of PNIPAM-PAAc microgel particles that makes this system a suitable and unique model to provide insights into the glass formation in molecular systems [45].
In this work, through DLS experiments, we study the dynamics of aque-ous dispersions of IPN microgels of PNIPAM and PAAc as a function of
temperature, pH, concentration and exchange wavevector Q across the VPT transition. The relaxation time τ shows a clear dependence on temperature and new features related to both concentration and pH. Furthermore the in-vestigation at dierent lengthscales has highlighted a τ ∝ Q−n dependence
with n >2. These results open the way to further investigations at dierent length and time scales.
2. Material and Methods 2.1. Materials
Materials. Both N-isopropylacrylamide (NIPAM) from Sigma-Aldrich and N,N'-methylene-bis-acrylamide (BIS) from Eastman Kodak were puri ed by recrystallization from hexane and methanol, respectively, dried under re-duced pressure (0.01 mmHg) at room temperature and stored at 253 K until used. Acrylic acid (AAc) from Sigma-Aldrich was puri ed by distillation (40 mmHg, 337 K) under nitrogen atmosphere in the presence of hydro-quinone and stored at 253 K until used. Sodium dodecyl sulphate (SDS), purity 98 %, potassium persulfate (KPS), purity 98 %, ammonium persul-fate, purity 98 %, N,N,N´,N´-tetramethylethylenediamine (TEMED), purity 99 %, ethylenediaminetetraacetic acid (EDTA), NaHCO3, were all purchased
from Sigma-Aldrich and used as received. Ultrapure water (resistivity: 18.2 MΩ/cm at 298 K) was obtained with Sarium® pro Ultrapure Water pu-ri cation Systems, Sartorius Stedim from demineralized water. All other solvents were RP grade (Carlo Erba) and were used as received. Before use dialysis tubing cellulose membrane, cut-o 14000 Da, from Sigma-Aldrich, was washed in running distilled water for 3 h, treated at 343 K for 10 min into a solution containing a 3.0 % weight concentration of NaHCO3 and 0.4
% of EDTA, rinsed in distilled water at 343 K for 10 min and nally in fresh distilled water at room temperature for 2 h.
Synthesis of IPN microparticles. The IPN microgel was synthesized by a sequential free radical polymerization method: in the rst step PNIPAM micro-particles were synthesized by precipitation polymerization and in the second step acrylic acid was polymerized into the preformed PNIPAM net-work [29]. (4.0850 ± 0.0001) g of NIPAM, (0.0695 ± 0.0001) g of BIS and (0.5990 ± 0.0001) g of SDS were solubilized in 300 ml of ultrapure water and transferred into a 500 ml ve-necked jacked reactor equipped with condenser and mechanical stirrer. The solution was deoxygenated by bubbling nitrogen
inside for 30 min and then heated at (273.0 ± 0.3) K. (0.1780 ± 0.0001) g of KPS (dissolved in 5 ml of deoxygenated water) was added to initiate the polymerization and the reaction was allowed to proceed for 4 h. The resultant PNIPAM microgel was puri ed by dialysis against distilled water with frequent water change for 2 weeks. The nal weight concentration and diameter of PNIPAM micro-particles were 1.02 % and 80 nm (at 298 K) as determined respectively by gravimetric and DLS measurements. (65.45 ± 0.01) g of the recovered PNIPAM dispersion and (0.50 ± 0.01) g of BIS were mixed and diluted with ultrapure water up to a volume of 320 ml. The mixture was transferred into a 500 ml ve-necked jacketed reactor kept at (295 ± 1) K by circulating water and deoxygenated by bubbling nitrogen inside for 1 h. 2.3 ml of AAc and (0.2016 ± 0.0001) g of TEMED were added and the polymerization was started with (0.2078 ± 0.0001) g of am-monium persulfate (dissolved in 5 ml of deoxygenated water). The reaction was allowed to proceed for 65 min and then stopped by exposing to air. The obtained IPN microgel was puri ed by dialysis against distilled water with frequent water change for 2 weeks, and then lyophilized to constant weight. The poly(acrylic acid) content determined by acid/base titration was 6.6 % weight concentration (PAAc/IPN).
Sample preparation. IPN samples were prepared by dispersing lyophilized IPN into ultrapure water at weight concentration 1.0 and 3.0 % by magnetic stirring for at least 3 h. Samples at dierent concentrations were obtained by dilution.
Characterization. The poly(acrylic acid) content in 10 g of IPN dispersion was determined by addition of 11 ml of 0.1 M NaOH followed by poten-tiometric back titration with 0.1 M hydrochloric acid. Concentration of dispersion was determined from the weight of the residuum after water re-moval by lyophilisation, corrected for the moisture residual amount obtained by thermogravimetric analysis (TGA). This was accomplished with a SII Nano-Technology EXTAR TG/DTA7220 thermal analyser at 275 K/min in nitrogen atmosphere (200 ml/min). 5 mg of sample in an alumina pan was analysed in the (313-473) K temperature range and the weight loss was as-sumed as moisture content.
2.2. Experimental Methods
DLS measurements have been performed with a multiangle light scat-tering setup. The monochromatic and polarized beam emitted from a solid
10-5 10-4 10-3 10-2 0 1 T= 313 K T= 311 K T= 309 K T= 307 K T= 305 K T= 303 K T= 301 K T= 299 K T= 297 K T= 295 K T= 293 K g2 (Q ,t )-1 t(s)
Figure 1: Normalized intensity autocorrelation functions of an IPN sample at Cw= 0.10
%, pH 7 and θ=90°for the indicated temperatures.
state laser with a wavelength λ=642 nm and a power of 100 mW is focused on the sample placed in a cylindrical VAT for index matching and tempera-ture control. The scattered intensity is collected at ve dierent values of the scattering angle θ=30°, 50°, 70°, 90°, 110°, that correspond to ve dierent values of the exchanged wavevector Q, according to the relation Q=(4πn/λ) sin(θ/2). Single mode optical bers coupled to collimators collect the scat-tered light as a function of time and scattering vector. In this way one can measure simultaneously the normalized intensity autocorrelation func-tion g2(Q, t) =< I(Q, t)I(Q, 0) > / < I(Q, 0) >2 at ve dierent Q values
with an high coherence factor close to the ideal unit value. Measurements have been performed as a function of temperature across the VPT.
In Fig. 1 the typical behavior of the normalized intensity autocorrela-tion funcautocorrela-tions for an IPN sample at weight concentraautocorrela-tion Cw=0.10 %, pH
7 and θ=90°for the indicated temperatures is shown. As commonly known the intensity correlation functions of most colloidal systems cannot be well described through a single exponential decay but the Kohlrausch-Williams-Watts expression [46, 47] is generally used:
295 300 305 310 315 0.6 0.8 1.0 shrunken state Ref. 35 C w=5 x 10 -6 g/mL 6.5< pH < 7 Ref. 46 0.15 < φ < 9.4) (pH dependence neglected) Our data C w=2.34 x 10 -3 g/mL pH 7 R (T )/ R (2 9 7 K ) T(K) swollen state
Figure 2: Normalized radius as obtained from DLS measurements for an IPN sample at Cw = 0.10 % (equivalent to a weight/volume concentration of 2.34 × 10−3g/mL), pH 7
and θ=90°compared with results from ref. [33, 45].
where b is the coherence factor, τ is an eective relaxation time and β mea-sures the distribution of relaxation times (associated with simple exponential decays). Most commonly, the dierent relaxation times present in glassy ma-terials lead to a stretching of the correlation functions and an exponent β < 1 (which here is referred to as stretched behavior). The relaxation time is related to the translational diusion coe cient Dt through the relation
τ = 1/(Q2Dt). In the limit of non interacting spherical particles the Stokes
Einstein relation R = KBT /6πηDtallows, known the viscosity η, to calculate
the hydrodynamic radius from the diusion coe cient.
In Fig. 2 the temperature dependence of R normalized with respect to the value at T =297 K obtained using the viscosity of the solvent is shown. The data are in agreement with previous works on the same system at dierent concentrations [33, 45]. The discrepancy at high temperature above the VPT is probably related to the approximation of the viscosity taken as that of the solvent. For these reasons in the following we will always refer to the behavior of the relaxation times.
0.0015 0.0020 0.0025 0.0030 0.0035 b) a) τ
(s
)
Cw=0.30% Cw=0.20% Cw=0.15% Cw=0.10% pH 7 295 300 305 310 0 1 β T(K)Figure 3: (a) Relaxation times and (b) stretching parameter from Eq. 1 as a function of temperature at pH 7 for the indicated concentrations. Full lines in (a) are guides to the eyes.
3. Results and Discussion
The DLS technique has been used to characterize the swelling behavior of the IPN microgel in the temperature range 293 K≤ T ≤ 313 K where the volume phase transition is expected to occur. In order to neglect interparti-cle interactions and avoid phase separation diluted solutions at four dierent weight concentrations Cw=0.10 %, Cw=0.15 %, Cw=0.20 %, Cw=0.30 %
have been studied. To test reproducibility measurements have been repeated several times. The pH dependence of the suspensions has been investigated both for acid and neutral solution at pH 5 and 7, respectively. In this way a clear picture of the microgel behavior as a function of temperature, pH and concentration is drawn. In Fig. 1 the intensity correlation functions show a clear transition occurring above T=305 K. The volume phase transition is better evidenced by looking at the relaxation time obtained through a t of the intensity correlation functions according to Eq. 1. Its behavior with temperature is reported in Fig. 3: as temperature is increased the relaxation time shows a slight decrease until the transition is approached around 305 K. Thereafter, depending on concentrations, dierent behaviors are observed.
0.0005 0.0010 0.0015 0.0020
β
pH 5 Cw=0.30% Cw=0.20% Cw=0.15% Cw=0.10%τ
(s
)
a) 295 300 305 310 0 1 b) T(K)Figure 4: (a) Relaxation times and (b) stretching parameter from Eq. 1 as a function of temperature at pH 5 for the indicated concentrations. Full lines in (a) are guides to the eyes.
For the lowest concentrated sample at Cw=0.10 % after 305 K the relaxation
time abruptly reaches its lowest value, corresponding to a transition of the microgel particles from the swollen to the shrunken state. The aforemen-tioned transition of IPN has been reported to be smoother with respect to the case of pure PNIPAM microgel due to the presence of the acrylic acid that makes the swelling capability of the microgel greatly reduced [29, 33, 34]. Our results indicate that at increasing concentration the jump becomes smaller and smaller with an interesting swap in trend for the highest concentrated sample at Cw=0.30 %. Moreover, at increasing concentration the relaxation
time increases. On the contrary the stretching parameter β does not show any change with temperature and concentration, remaining just below one, indicating correlation functions slightly stretched.
This behavior is strongly aected by the pH of the solution as shown in Fig. 4 where relaxation time and stretching parameters as a function of temperature under acid conditions, at pH 5, are reported. At variance with neutral pH solutions in this case a sharp transition is never observed and as temperature is increased the relaxation time always decreases. Furthermore
10-5 10-4 10-3 10-2 10-1 0 1 Q = 6.7 x 10-3 nm-1 Q = 1.1 x 10-2 nm-1 Q = 1.5 x 10-2 nm-1 Q = 1.8 x 10-2 nm-1 Q = 2.1 x 10-2 nm-1 g 2 (Q ,t )-1 t(s)
Figure 5: Normalized intensity autocorrelation curves at Cw=0.20 %, T=303 K and pH 7
for the indicated values of the exchanged momentum Q.
an opposite trend with respect to concentration is observed: with increasing concentration the relaxation time decreases. The comparison between Fig. 3a and Fig. 4a shows also that the relaxation times are always smaller in acid conditions. As in the case of neutral pH the stretching coe cient β is neither temperature nor concentration dependent and is always slightly below 1.
Therefore the pH of the solution strongly aects the relaxation times behavior: at neutral pH its values are higher and the transition appears to be more evident than in the case at acid pH. This dependence of the transition not only on concentration but also on pH con rms that the eect observed is due to the presence of PAAc. On the contrary the stretching parameter is insensitive even to pH changes.
To investigate the nature of the motion and to obtain information on dierent lengthscales we have studied the wavevector Q dependence of the relaxation time and stretching parameter. In Fig. 5 the normalized intensity correlation functions collected at dierent scattering angles for a sample at Cw=0.20 %, T=303 K and pH 7 are reported. The behaviors of relaxation
time and stretching parameter as obtained through ts of the autocorrelation curves according to Eq. 1 are shown as a function of the wave vector Q in
0.01 0.015 0.02 1 ββββ Q(nm-1) T=311 K T=307 K T=303 K T=299 K T=295 K b) 10-3 10-2 τ
(s
)
a)Figure 6: (a) Relaxation time and (b) stretching parameter from Eq. 1 as a function of momentum transfer Q at Cw=0.10 %, pH 7 for the indicated temperatures. Full lines are
ts through Eq. 2 with n=2.33 ± 0.03 (T=311 K), n=2.31 ± 0.07 (T=307 K), n=2.465 ± 0.008 (T=303 K), n=2.81 ± 0.08 (T=299 K), n=2.70 ± 0.03 (T=295 K).
Fig. 6.
The relaxation time, reported in a double logarithmic plot, is strongly Q dependent, with a typical power law decay described by the relation:
τ = AQ−n (2)
where A is a constant and the exponent n de nes the nature of the motion. In Fig. 6a the ts according to Eq. 2 (full lines) are superimposed to the data (symbols) and values of n >2 (in particular n between 2 and 3) are found, in agreement with those reported in previous studies on the same microgel [45] and on dierent polymers [48, 49].
In Fig. 6b the stretching parameter β is reported, clearly showing no dependence on the wave vector Q.
4. Conclusions
The swelling behavior of the PNIPAM-PAAc IPN microgel has been stud-ied as a function of temperature, pH, concentration and wavevector. The
presence of PNIPAM within the network determines the temperature sen-sitivity behavior and the addition of PAAc introduces an additional pH-sensitivity leading to interesting dierences in the transition process at acid and neutral pH, respectively. In particular we have found that the volume phase transition is more pronounced at pH 7 with respect to pH 5 showing a discontinuous and a continuous behavior respectively. Furthermore, while at neutral pH the relaxation time reaches the highest value at the highest concentration, at acid pH the dependence is inverted. Finally at neutral pH the relaxation time for the highest concentrated microgel shows, at odds with the low concentration samples, an intriguing increase after the VPT transition. This could be a precursor of the more complex behavior expected at even higher concentrations where a thermoreversible gelation should be observed [45]. It would be very interesting to address how the swelling capa-bility and the gelation depend on PAAc concentration.
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